1. Field of the Invention
The present invention relates to a method for manufacturing a rotary cutting tool for forming transverse slits in a sheet such as a paper sheet, a plastic sheet, a metal sheet or the like, and a rotary cutting tool manufactured by the method.
2. Description of the Related Art
Rotary cutting tools are used for forming fully penetrated slits in which cutting edges fully penetrate or pass through a sheet or partially penetrated slits in which cutting edges partially penetrate the sheets, i.e., the depth of the cuts is smaller than the thickness of the sheets to be slitted. The slits formed in the sheet can be used in a variety of ways. For example, the sheet can be easily separated or folded at the position of continuous or discontinuous slits. The slits are formed in a staggered pattern and the sheet is then longitudinally spread to obtain a sheet having a network structure, or the slits are formed in a stretchable plastic sheet and the sheet is then transversely stretched to obtain a sheet having a network structure having high strength in the transverse direction. When the slits are fully penetrated slits and the interval between the longitudinally adjacent rows of slits is relatively large, a shearing cutter can be used. However, when the slits are partially penetrated slits and/or the interval between the longitudinally adjacent rows of slits is relatively small and many slits have to be efficiently formed, an accurately finished rotary cutting tool is used, then the sheet is pinched and conveyed between the rotary cutting tool and a backing roll.
For example, there is a known nonwoven fabric which comprises the lamination of a first sheet having transverse ribbons and a second sheet having longitudinal ribbons arranged so that the transverse ribbons and the longitudinal ribbons intersect with each other as if they are weft and warp of a woven fabric. The first sheet is fabricated from a sheet of plastic film, by forming transverse slits in the sheet with a rotary cutting tool having cutting blades and then laterally stretching the sheet. The second sheet is also fabricated from a sheet of plastic film, by longitudinally streching the sheet, splitting the sheet to form longitudinal splits or slits with another rotary cutting tool and then laterally spreading the sheet. Here, the word "longitudinal" means the longitudinal direction of the continuous sheet or the conveying direction of the continuous sheet. The present invention relates to the rotary cutting tool for forming the first sheet having the transverse slits, although it is not limited to the illustrated example.
The manufacture of the rotary cutting tool for fabricating the sheet having many transverse slits is very difficult because a large number of cutting blades are arranged on the outer circumferential surface of the rotary cutting tool at a small circumferential pitch. For example, the rotary cutting tool includes cutting blades arranged on the outer circumferential surface of the rotary cutting tool at a circumferential pitch of 3.0 millimeter or less.
There are two types of known rotary cutting tools for fabricating the sheet having transverse slits. One type of rotary cutting tool comprises a cylindrical body and cutting blades which are integral with the cylindrical body. That is, the cutting blades are formed one by one by machining or cutting an outer circumferential portion of the cylindrical body. After the rotary cutting tool is roughly machined, a hardening treatment, for example, cementation, is applied to the rotary cutting tool, and it is manually finished while observing the cutting blades through a microscope. To manufacture this rotary cutting tool, an artisan must have enough experience and high skill, and yet a considerable time is necessary to complete one rotary cutting tool. In addition, the material of the rotary cutting tool should not be so hard because it must be machined, and such a material does not ensure a long operation life of the cutting blades of the rotary cutting tool.
Another type of rotary cutting tool comprises a cylindrical body having grooves arranged in the outer circumferential surface generally parallel to the axis of the cylindrical body, and cutting blades fitted in the grooves. In this case, it is desirable that the cutting edges of the cutting blades are accurately arranged on a circle about the axis of the cylindrical body, but in practice, the cutting edges of the cutting blades are not accurately arranged on a circle about the axis of the cylindrical body. There is a deviation in the heights of the cutting edges from the axis of the cylindrical body, and such a deviation is larger than an allowable level. That is, the grooves of the cylindrical body are machined and there is a deviation in the bottom surfaces of the grooves from the axis of the cylindrical body, but such a deviation is inevitably large because accurate machining cannot be ensured for narrow and dense grooves. The cutting blades themselves contain a certain deviation and a deviation of the cutting edges of the cutting blades is added to a deviation of the grooves when the cutting blades are fitted in the grooves of the cylindrical body. To solve this problem, the depths of the grooves and the heights of the cutting blades are measured, respectively, and appropriate cutting blades are selected for particular grooves so that a deviation in the heights of the cutting blades when they are fitted in the grooves becomes smaller, but a deviation in the heights of the cutting blades is still larger than an allowable level. To mitigate the deviation in the heights of the cutting blades, it is conceived that the cutting blades are adjustably fixed in the grooves, using adjusting device, shim screws and so on. However, in the case of the rotary cutting tool having a large number of cutting blades arranged at a small circumferential pitch, it is not possible to arrange and use adjusting screws because the area or space per groove is very small.
It is needless to say that the cutting edges of the cutting blades must be accurately arranged relative to the axis of the rotary cutting tool when the slits are partially penetrated slits and the sheet is thin. It is necessary too that the cutting edges of the cutting blades must be accurately arranged when the slits are fully penetrated slits or when the slits are arranged at a relatively large pitch. If the cutting edges of the cutting blades are not accurately arranged, the slits are not exactly formed and it is necessary to increase a pressure applied between the rotary cutting tool and the backing roll. However, if an excessive load is applied, operating life of the cutting blades and the backing roll is reduced. Therefore, the cutting edges of the cutting blades must be accurately arranged when the slits are fully penetrated slits.
When the rotary cutting tool is of the integral type in which the cutting blades are integral with the cylindrical body and formed by machining the outer circumferential portion of the cylindrical body, high skill and a great deal of labor are necessary, as described above. In addition, if only a part of the cutting blades is damaged during the manufacture of the rotary cutting tool or in use, it is sometimes difficult to re-arrange or re-machine the cutting blades depending on an extent of damage, and accordingly a new rotary cutting tool must be manufactured. In particular, when the sheet is a thermoplastic plastic sheet which is stretched in a heated condition after slits are formed in the sheet, a material of the sheet having the slits may be broken during stretching if there are defective slits formed in the slit forming process, and in this case, the cutting blades are sometimes heated above a melting point of the plastic sheet in the slit forming process to avoid this problem. However, the cutting blades are subjected to a thermal deformation, so it is difficult to maintain a desired accuracy of the cutting blades in the heated condition. It is also difficult to finish the cutting tool, expecting that the cutting edges can maintain high accuracy in the heated condition.